The development of short pulse optical communication systems or long range optical imagers for use in the oceanic transmission channel requires accurate knowledge of the effects of absorptive, particulate multiple scattering on the transfer of optical radiation. Generally speaking, this multiple scattering process inhibits optimum system performance by inducing such things as an additional beam spread, energy loss, a degradation of spatial coherence, depolarization, and a dispersion in time and frequency of the signal modulating the initial radiance distribution. Hence, a receiver is compromised by having increased design complexity with diminished performance in return for increased operational range. An in-depth study was presented by R. C. Honey and G. P. Sorenson and entitled "Optical Absorption and Turbulence-induced Narrow-angle Forward Scatter in the Sea" at the AGARD Conference No. 77 on "Electromagnetics of the Sea", AGARD-CP-77-70; November 1970. Based on this article, the optical absorption appears to be the limiting factor in the optimum performance of any underwater system. That is, the light absorbed by the water and particulate matter in the water is not merely redistributed but is transformed into heat and is lost as prospective information.
The absorption mean free path for the "blue-green" window ranges from a few tens of centimeters or less in dirty harbors to tens of meters in very clean ocean waters. At other wavelengths, the absorption mean free path is much shorter. Thus, even in clear ocean water, optimum system performance is limited to just a few hundred meters of operational range. For example, if a 1000-joule laser pulse is transmitted through 30 meter water and no scattering occurs, then only a single photon will be left after a range of 1.5 km. Increasing the signal power by an order of magnitude only extends the above range by 70 m. Coupling these pulses with normal imaging/communications requirements, e.g. having a minimum system signal-to-noise ratio in view of the inherent background noise sources in seawater, just further increases the signal energy requirements by several orders of magnitude, even in the clearest of ocean waters. Unfortunately, because of the foregoing constraints, electro-optical technology limitations generally force the proposed system's operational range to be reduced. Thus, an accurate determination of real-world volume absorption coefficients is mandatory for a true assessment of a system and its importance to this assessment cannot be overestimated.
The importance of being able to determine the true absorption coefficients becomes of greater significance when a practical application of optical communication or ranging is to be attempted in the field. At present, no in situ measurement techniques exist for determining the true absorption coefficient "a" of particulate media under field conditions. Several methods have been proposed as noted in the article identified above, but are generally limited to very homogenous and static paths. No known approach addresses itself to a method for determining the true absorption coefficient in situ of both homogenous and inhomogenous dynamic particulate scattering media, e. g. several different volumes of ocean water, over extended paths using pulsed laser techniques. This capability needs further manifestation to provide researchers and designers with necessary design parameters from which devices can be fabricated to enable more reliable undersea communications.
Thus, there exists a continuing need in the state-of-the-art for an in situ method for determining the absorption coefficient of pulsed light energy through a variety of long-path media which is capable of a nearly real-time analysis by moderately skilled technicians.